Photo-exposure meter.



S. PRATT.

PHOTO EXPOSURE METER. APPLICATION FILED APR. 2a, 1908.

910,230. Patented Jan. 19, 1909.

2 SHEETS-SHEET 1L W/TNESSES INVENTOH ATTOHNFYS S. PRATT.

PHOTO EXPOSURE METER.

APPLICATION FILED APR. 28, 190B.

Patented Jan. 19, 1909.

2 SHEETS-SHEET 2.

AFTERNOON HOl/RSSOUTHERN HEMISPHERE. MORNING H0025 NORTHERN HEMISPHERE.

AFTE

MORNING ICA IJU INVENTOR edgwz'ck'fraii mrd A 7TOHNEYS UNITED STATES PATENT orrion.

SEDGWIGK PRATT, OF PASADENA, CALIFORNIA.

PHOTO-EXPOSURE METER.

Specification of Letters Yatent.

Patented'J'an. 19, 1909.

Application filed April 28, 1908. Sei'ial No. 429,697.

To all whom it may concern:

Be it known that I, Snoewrox PRATT, a citizen of the United States, and a resident of Pasadena, in the county of Los Angeles and State of California, have invented a new and Improved Photo-Exposure Meter .Figp3 is a pla which rests upon he bottom of the box, this .35

of which the following exact description.

My. invention relates to instruments for measuring the degree of exposure to light, for instance in photography, my more particular purpose "being to provide a hotois 'a full, clear, an

exposure meter having quite a num' er of uses and involving a minimum of mechanical parts.

Reference is to be had to the accompanying drawings forming a part of this specifirst mentioned; Fig. 2 is a section upon the line 22 of Fig. 1, looking in the direction of the arrow and showing the mechanical construction of the box and its contents; view of the circular card view further showing a number of lines,

curves and graphic symbols wherewith the.

card or statlonar member of the device is provided; and Flg. 1 is a plan view of the transparent disk, made preferably of celluloid and mounted within the box, this disk.

being revoluble relatively to the circular card and being provided with lines, graphic symbols and other indicating members havingva predetermined relation to various lines and graphic symbols 'of the card.

A metallic b0x'5is rovided with a back 6 and is further provi ed with an overhangi flange 7 the latter being secured in posit1on by aid of a rim 8 integralwith it. Mounted within the box and secured against the back 6 is a circular card 9. A disk 10 of celluloid is mounted revolubly within the box and disposed in loose engagement with the card 9. A disk 11 of glass is secured to the celluloid disk 10 and is provided with a thumb piece 12 whereby both of these members, considered as a unit, may be turned relatively to the circular card 9. Y

Extending diametrically across the celluloid disk lO-is a line 13 having adjacent to its central point the numeral 1. This line, in some of the uses to which the instrument is put, designates the horizon, as hereinafter explained. Disposed upon opposite sides of the line representing the horizon are a number of other lines 14 parallel with each other but spaced at unequal distances from the line 13, as will be understood from Fig. 4. These lines 14 are provided with various numerals from to 800 and between the numeral 60 and the numeral 1 of the line 13 is the numeral 30. The numerals 30 to 800 are duplicated; that is, they extend in two directions from the center,'as will be understood from Fig. 4. The numbers 1 to 800 represent what I designate as a diaphragmtime scale.

The celluloid disk is further. provided with a pointer 15 having preferably the form of an arrow, as shown in Fig. 4, this pointer being immovable relatively to the celluloid disk. The circular card 9 is provided with a circle '16 and disposed adjacent to a portion of the latter are aduations 17 ar-. ranged in a row of su stantially' arcuate form,'as will be understood from the u per portionof Fig. 3. Disposed adjacent to t ese graduations are numerals 0 to inclusive,

constituting what I designate as a latitude scale.

Immediately inside of the circle 16 and disposed adjacent to the latitude scale just mentioned, are numeralslv to 800, indicatlng, as in the disk 10, a diaphragm-time scale. The circular card 9 is provided with a numher of lines 18 parallel with each other but V spaced apart by' une ual distances. I designate these lines as date lines. Crossing their general direction and disposed close to. their ends are various dates ran 'ng from June 21 to December 21, and from ecember 21 to June 21, these dates being'arranged incolumns- 19, 20, which, for convenience, I'

designateas date columns.

A number of lines 21, some curved more than others, are placed upon :(pposite sides at unequal of a central line 22 and s a distances'therefrom. The ines 21,122 represent hours and. are provided, at their upper ends, and also at their lower ends, with-numerals representing hours. While these numerals run only from 1 to 11, it will be unmorning hours Northern Hemisphere.

by ascertalning the product of the sd-Called stop number used in the camera to be. employed, and multiplying the same by the reciprocal of the corresponding time of exposure for normal conditions and for the particular latitude and time of day. For abnormal conditions some allowance should be made by multiplying a proper factor. These-called stop number to be employed in making an exposure, divided by the reading of the diaphragm-time scale, as hereinafter pointed out (or as corrected for abnormal conditions) will be the required time of exposure in seconds.

The construction of the date lines 18 and of the other lines spaced equidistant and used upon either the circular card 9 or the celluloid disk 10, are determined mathematically. These being mere details to be employed in practical construction, I do not deem it necessary to describe them.

The uses to which my invention can be placed are ma'nyand varied. For the sake of simplicityfll herein describe only afew of them.. In describing the use of the instrument, the stop numbers referred to will be understood asthe'ordina'ry stop numbers relied upon in the use of cameras and as having a conventional designation given them byphotographers in actual practice.

The particular use of the diaphragm-time scales may be best understood by a particular example: Suppose it be desired that we find the time of exposure when the latitudeis not known and also when the time of day is not known. This may be done by turningthe disk 10 until the line 13 "(represenu.

ing the horizon) reaches the 90 mark of the latitude scale *(upper left hand in Fig. 3). The operator now holds the instrument in his left hand at the height of the eye,

maintaining the line 13 vertical. The sun 18 now to the right and just shining upon the face of the instrument. If a straight edge such as a pin or a match, be now' moved along perpendicular to the right side of the instrument until the shadow of the straight edge passes exactly through the center of the instrument, the operator may read the diaphragm-time scale (indicated by the provided with the following legend numbers 1 to 800), and taking the approximate number where the shadow cuts this scale and dividing the stop number by this .I'iu-inbe'r, the time of exposure is ascertained.

The operation of my device is as follows: Suppose the operator is' in latitude 10 north, that the date is August 21, the time of day 9 a. m and that the normal conditions obtain. Suppose further, that the particular stop to be used isfthe one designated ordinarily as No. 10, U. S. The question is, what should be the time of exposure for the conditions just stated? Tn order to solve this questionfthe operator moves the thumb p'ece 12, and consequently turns the celluloid disk 10 and glass; plate 11 (these two members acting as a unit) so that the arrow 15 points to the numeral 40 in the latitude scale in the upper portion of the card 9, as will be understood from Fig. 1. Selecting from the date column19 the date in guest-ion, to wit; August 21, the operator follows the date line associated with this date, and finds where this line meets the particular hour curve corresplondmg to'9 a. In. of the Northern Hemisp ere. From the point constituting the junction of these two lines he now ascertains the nearest of the parallel lines 13, 14c upon the celluloid disk, and having done this he follows the particular line until it reaches the diaphragm-time scale (indicated by numerals 1 to 800), andreads the nearest number of the scale. -This he finds to be 380. Now, dividing the stop number 10 by the reading thus ascertained, he has 10 divided by 380, which gives 3 3;. The required time of exposure is therefore of a second. Suppose, however, that the conditions laid down are not-quite the same as those just described, there being avariation in the fact that instead of a nor- .mallandscape the subject to be photographed is a portrait which is located in the shade of a building without any overhead cover. In this case we should find, as'above, the dia hragm-time reading, namely, 380, and mu tiply by a modifying factor, say a,

this being an arbitrary allowance, supposedly sufficient to cover the difference in the degree of light thrown upon the portrait, and that which should be thrown upon a normal landscape. his gives 380 multiplied by equal to 76, and dividing the sto number 10 by 76 we have of a secon nearly, as the time required for exposure. Examples analogous to the two just stated arise qulte frequently in practice. There are also a. number of other examples which I state for the sake of clearness, as follows: Example. 3: Latitude 34 north; date, November 6; time of day 10.30 a. m.; stop number 4 U. S. sun dimmed by light clouds Hammerslow plate to be used. What would be the time of exposure? Solution: Set the instrument so that the the time of day, to wit, 10.30, which, if

drawn, would meet a parallel line half way between the parallel lines designated as 250 and 380. We now findthat the diaphragmtime reading is 250 plus 380, all divided by 2 equaling 315. Multiplying this by a so as to allow for the fact of the sunbeing dimmed by clouds, and by 0.3 for thefuse of the Hammerslow plate, we have 315 multiplied by multiplied by 0.3, equals 38 nearly, and dividing 4 by this number we resent 7 .25 p. m. or sunset.

have nearly, as the fraction of a second of exposure required.

Example 4: Date, June 21; latitude 40 north. What is the mean time (approximately) of sunrise and sunset? 5 Solution: Set the meter so that the hand 1.3 points to number 40 in the latitude scale, meaning 40. Follow the date line of June 21 to a point where it intersects the parallel line through the center of the meter. This parallel line, as above explained, represents the horizon. The hour curve at the intersection of the two lines would be 4.35 a. m. This would represent the time. of sunrise. The other end'of the hour curve would rep- Example 5: At what latitude is the sun overhead at noon on June 21, in the Northern Hemisphere? Solution: Turn the disk 10 until the parallel 800 is tangent to the circle 16 at the point indicating June 21. Now read, the latitude as indicated by the arrow; the an swer will be found to be 234. In this connection it may be noted that the parallel lines form practically a scale of altitudes of the sun and are 10 apart, the zero of the .scale being, of course, the line 13 representing the horizon and extending directly across the center of the disk.

Example 6: At 66 north latitude, what is the2 altitude of the sun at noon on December 21 Solution: Set the disk 10 to 66 latitude. The line 13, representing the horizon, now passes through the designation December 21 at the point representing noon. The answer, therefore, is 0.

Example 7 At 30 south latitude, what is the altitude of the'sunat 9 a. m.. March. 11?

.Follow the March 11 date line to the 9 Solution: Set the disk to 30 latitude.

where the parallel 380 will be found to pass.

In the latitude scale (which relatively to p the diaphragm-time scale below it is an altitude scale) 40 corresponds to 380. The altitude is therefore 40.

Example 8: Where and when are sunrise and sunset at the same time? Solution: The instrument shows this'willbe the case everywhere on March 21 and September 21, and always at 0 latitude.

Example 9 At what latitude in the Northern Hemisphere will the time of exposure be the same at any time of day during the24 hours the same date, under the same conditions? Answer: 90. During what months is this true? AnswerrFrom March 21 to September 21. And on July 21, using stop number 10 U. S., normal conditions, what should be the exposure? this exposure should be givenat 7.25 a. in.

and at 4.35 p. m.

The following directions may be conven-- hour curve, and from this'point follow red Answer: orof a second. At the equator on this date,

line or parallel thereto in Northern Hemisphere, and black line in Southern Hemie sphere) till scale on glass is met. The stop number (uniform system) to be used divided by the scale reading will give the time of exposure for normal conditions.

Normal conditions. Open landscape; open foreground; no dark objects near camera; clear sky; N. C. films. For other conditions multiply scale reading as follows: Average landscape, light foreground, small dark objects in foreground; buildings 30 to 50 feet away; foliage, figures, etc., at mid-distance:

Multiply by a or 0.5. Dark objects or portraits out doors in shade, no overhead cover: Multiply by or 0.2. Same and overhead cover, multiply by or 0.1. Snow view; mid-sea. view; heavy clouds; far-distant landscapepMultiply by 1% or 1.25. Distant sea view, light clouds, multiply by 3% or 3.5. Sun dimmed by light clouds or haze: multiply by to Seeds 27 and similar plates, multiply by,%% or 1.1. Seeds 26,'X, etc., multiply by or 0.7. Seeds23, etc., multily by 4 or 0.5. Hammerslow, etc., multiply Having thus described my invention, I claim as new and desire to secure by Letters Patent: I

1. The combination of a card provided with lines, a disk of transparent material mounted upon said card and revoluble in relation thereto; said disk being provided with lines adapted to be brought into regj,-,s try with said lines upon said card, said disk being further provided with numerals for as a pointer and adapted to be brought into 1 registry with predetermined portions of said card and a box for receiving the card and mounted upon the card and rotatable with.

respect thereto, and provided with lines V adapted to be brought into register with difi'erent portions of the latitude scale, the rim of the box being provided with an overhanging flange for retaining the disk and 15.

I the card in place.

i In testimony Wher'eof'I have slgned my name to this specification in the presence of two subscribing witnesses.

SEDGWVICK PRATT. Witnesses:

JOHN MCDONALD,

MIRIAM LEE. 

